1. Field of the Invention
The present invention relates generally to an optical disc drive for reproducing content from either continuously recorded media or sector format media, and relates more specifically to an optical disc device for reading content with few errors from an optical disc having black dots, scratches, or other imperfections causing read errors.
It should be noted that Japanese Patent Application 11-207601 and U.S. Pat. No. 6,104,682 relating to the present invention are incorporated herein by way of reference in their entirety.
2. Description of Related Art
Discs, such as DVD-ROM discs, that record data continuously, and discs, such as DVD-RAM discs, having a sector format and recording data by sector, are both commonly used today. Signals are generally recorded to continuous-recording disc media so that the playback signal is DC-free. “DC-free” represents the average signal level of the playback signal obtained from the pattern recorded to the medium being always constant within a certain data length (such as one byte).
A digital sum value (hereinafter referred to as DSV) can be used to evaluate DC-free performance. The DSV is the sum of signs within a specific data period where positive (+) polarity is assigned to the mark side, and negative (−) polarity is assigned to the gap (that is, the unrecorded part between marks) side of the playback wave obtained from the recorded signal pattern. If a signal is completely DC-free, the DSV will be 0, and if the DSV of a particular byte is 0, the byte is considered to be DC-free.
By recording continuously recorded data so that the playback signal is generally DC-free, it is not necessary to consider DC level fluctuation when the playback signal is processed after analog-to-digital (hereinafter referred to as A/D) conversion. A conventional optical disc drive thus only needs to control the DC level of the input signal to the A/D converter so that the DSV of the A/D converted signal is 0.
When the playback signal is obtained from sector format media such as DVD-RAM discs, however, there is a sharp change in the DC level between the playback signal from the prepit address area of the disc and the playback signal from the data content storage area of the disc. There is also no assurance that signals are recorded in all areas because recorded sectors and blank sectors are intermixed.
Conventional disc drives control the DC level of the input signal to the A/D converter so that the DSV of the signal after AND conversion is 0.
DC level convergence where the DC level changes greatly and abruptly at the change between the prepit address area and the content recording area is, however, time consuming.
The gain crossover point can be raised to speed this convergence, but this technique adds noise to the data after AID conversion. Another method of reducing the effects of DC level fluctuation is to first pass the signal through a high-pass filter before A/D conversion. However, when an area where there is a scratch or black dot or other imperfection on the disc is reproduced, the signal level changes greatly after passing the scratch or black dot, for example, and may exceed the tolerance range of the A/D converter. Recovering the playback signal in such cases takes time.